Right now, the standard openerp web service is based on the objects ( partner, sales order etc.). The operations are read, write, update,delete. The default methods just bring the values as it is in case I use the read method on a partner object. But I would like to override the method and do some changes. So that , the users of my webservice get the right data and in the right format

Yes. That will work. People will find your answer useful for straight forward use. But, If I have to pass over the json result in a particular format, Just overriding the method will not help for my strange requirement I think. Because OpenERP passes the data over in json format. That is not the desired format for me. I want it to be in a different data structure. I should do that only in the osv file. Correct me if I am wrong.